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MINI 33 - Radiotherapy and Complications (ID 164)
- Event: WCLC 2015
- Type: Mini Oral
- Track: Treatment of Locoregional Disease – NSCLC
- Presentations: 1
MINI33.09 - Impact of Tumor Regression and Need for Re-Plan during Radiation Therapy for Stage IIIB Lung Cancer: Dosimetric Comparison between IMRT and IMPT (ID 3063)
18:30 - 20:00 | Author(s): C. Hong
Radiation pneumonitis (RP) is the most worrying complication following high dose radiation therapy (RT) for loco-regionally advanced lung cancer. Intensity modulated radiation therapy (IMRT) and intensity modulated proton therapy (IMPT) are expected to reduce RP compared with conventional RT technique. Adaptive re-plan is usually indicated to accommodate tumor shrinkage and position changes during fractionated RT course. This study is to comparatively evaluate dosimetric differences between RT techniques and interval changes of these parameters assuming that initial IMRT and IMPT plans are continued throughout RT course without adaptive re-plan.
Ten patients who were given concurrent chemo-radiotherapy by IMRT (66 Gy/33 fractions, weekly Docetaxel/CDDP #6) for having N3(+) stage IIIB lung cancer were selected. Surrogate rival IMPT plan on each patient was generated to compare with initial IMRT plan. Beam numbers used in IMRT and IMPT were 6-7 and 3. Second CT obtained during 3[rd]-4[th] week for adaptive IMRT plan was used to generate second sets of IMRT and IMPT plans, assuming that adaptive plan had not been done. Differences between initial RT techniques and changes in dosimetric parameters including conformity index (CI), homogeneity index (HI) and dose-volume histogram (DVH) of target and normal organs, which could have occurred by 2 RT techniques, were compared.
When comparing initial IMRT and IMPT plans, IMPT showed advantageous features over IMRT with respects to median HI (1.08 vs. 1.02), mean doses (D~mean~) to lung, esophagus, and heart, lung volumes receiving 5 Gy (V~5~), 10 Gy (V~10~), 20 Gy (V~20~), 30 Gy (V~30~), and 40 Gy (V~40~) and maximum dose (D~max~) to spinal cord (all p<0.05), respectively. Mean gross tumor volumes (GTV) on initial and second CT’s were 90.9 (48.1~163.7) cm and 52.2 (23.1~89.7) cm and median GTV reduction was 42.2% (51.3%-84.4%). More dosimetric parameters could have changed significantly by IMPT (CI, HI, V~5~, V~10~, V~20~, V~30~, D~mean~ to lung and heart, and D~max~ to spinal cord) than IMRT (CI, HI, V~20~, D~max~ to spinal cord and heart), respectively. Absolute increase in D~max~ to spinal cord was estimated as 0.53 Gy by IMRT and 4.79 Gy by IMPT (p=0.003).
Impact of GTV regression during RT course and need for adaptive re-plan seem evident. More uncertainties on dosimetric parameters and higher doses to spinal cord are expected by IMPT than by IMRT if re-plan is not applied. Optimal timing and frequency of adaptive plans, however, need to be further evaluated.
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